Rangefinder- A device that measures distance from the observer to a target, for the purposes of surveying, determining focus in photography, or accurately aiming a weapon.

Parallax- An apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.

Pentaprism-a five-sided reflecting prism used to deviate a beam of light by 90°.

Viewfinder- What the photographer looks through to compose, and in many cases to focus, the picture.